Re: [Xen-devel] Hypervisor cpuid time leaf

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 03/05/17 14:57, Boris Ostrovsky wrote:
> Is there a reason why we don't document hypervisor time leaf (3, or is
> it 4?) in public/arch-x86/cpuid.h?

(The leaf with the number 3.  The way the documentation refers to leaves
and numeric values is very counter-intuitive.  I half remember a plan to
renumber the comments to be zero-based, which then match the constants. 
Also, XEN_CPUID_MAX_NUM_LEAVES is entirely erroneous to have in the
public API.)

I wondered the same when I looked at it.  I presume it was due to
insufficient review of the virtual timing modes when they were introduced.

>
> We have a regression in Linux where there is a window when
> vcpu_time_info data is not yet available and one possibility is to use
> this leaf. But I'd like to be sure it is part of a stable ABI.

One problem it has is that there is no indication of the valid subleafs
(a problem shared with the subsequent leaf).  I'd like to get agreement
on how to sort that (possibly via documentation only) before declaring
the ABI stable.

~Andrew

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